A micro-porous polyolefin film has been widely used as a battery separator, a separation filter, a micro-filtration separation membrane, or the like, due to excellent chemical stability and superior physical properties thereof. Among them, a separator for a secondary battery has a high level of ion transfer power through inner pores together with a spatial separation function between a cathode and an anode. Recently, the demand for improving characteristics of the separator has been increasing in order to improve electric stability of the battery due to higher capacitance and higher output of the secondary battery. In the case of a lithium ion secondary battery, the degradation in thermal stability of the separator may cause damage or deformation of the separator due to the increase in temperature caused by abnormal behavior of the battery and the accompanying short circuits between electrodes, and further may have risks of overheating, ignition, or explosion of the battery.
Recently, in the conditions where high output/high capacitance of the battery are requested, such as IT, electric drive vehicle (EDV), electric tool, energy storage system, and the like, the possibility of ignition and explosion of the battery generated at the time of abnormal behavior is several times to several tens of times that of the existing battery, and thus, high-temperature thermal stability enabling preparation against the increase in temperature of the battery is acutely needed. The separation having excellent high-temperature thermal stability prevents damage thereof at a high temperature, thereby blocking direct short circuits between electrodes. The battery is overheated when short circuit between electrodes occurs due to foreign materials, such as, dendrite and the like generated in the electrodes during the charging or discharging procedure of the battery. Here, the separator having excellent high-temperature thermal stability prevents fundamental damage thereof, thereby suppressing ignition, explosion, and the like thereof.
Korean Patent Laid-Open Publication No. 2007-0080245 and International Patent Publication No. WO2005/049318 disclose that a polyvinylidene fluoride copolymer, which is a heat-resistant resin, is introduced to a coating layer to thereby improve heat resistance of the separator and thermal stability of the battery, but have limitations in improvement in thermal stability of the battery since an heat-resistant resin in a non-aqueous battery or the like is easily dissolved or gelled in an electrolyte.
Japanese Patent Laid-Open Publication No. 2002-355938 discloses a micro-porous polyolefin composite film using a high heat-resistant resin. The high heat-resistant resin is introduced to the micro-porous polyethylene film by a phase separation method. However, it is difficult to realize efficient permeability by employing a method of using a single resin to form a thin film coating layer and performing a drying process to form pores therein through phase separation. Further, the size and uniformity of phase separation are largely varied depending on the drying conditions such as humidity, temperature, and the like, which has limitations to producing a separator having excellent uniformity in product quality. Moreover, shrinkage of a base layer due to a sharply increase in temperature caused by an abnormal behavior of the battery, such as internal short circuits of the battery, may not be effectively blocked due to its loose structure. Since the coating layer itself is not thermally deformed at 130° C., which is almost a melting point of the base layer, due to excellent heat resistance thereof, the shrinkage of the base layer may be partially blocked. However, the heat resistance of the coating layer is too small to completely block the shrinkage of the base layer due to low permeability and a loose net structure of the coating layer, and thus, this method is not suitable for forming a separator having improved thermal stability.
As most methods for improving heat resistance, there is provided a process of forming the coating layer of a heat-resistant resin using an organic solvent. Here, a large amount of organic solvent is used in order to dissolve the heat-resistant resin. The use of the organic solvent leads to deterioration in economical feasibility due to collection of the solvent or incineration after coating and drying of the organic solvent, and the organic solvent is not ecofriendly. In addition, the organic solvent has excellent affinity with polyolefin constituting the micro-porous polyolefin film, and thus is absorbed into the micro-porous polyolefin film during an application process. The heat-resistant resin is applied into the pores of the micro-porous polyolefin film by using an organic solvent where the heat-resistant resin is dissolved, after a drying process. The micro-porous polyolefin film applied with the heat-resistant resin reduces sizes of the pores due to the heat-resistant resin, resulting in decreasing the permeability. In addition, when a shut-down function of the micro-porous polyolefin film is expressed at a high temperature, the shut-down function of the micro-porous polyolefin film may be inhibited by the heat-resistant resin coated into the pores. As such, the use of the organic solvent for improving heat resistance may be environmentally problematic and cause basic functions of the micro-porous polyolefin film to be deteriorated, which cancels out advantages that is to be obtained by applying a heat-resistant layer. Moreover, even in the case of using the heat-resistant resin, long-term swelling and melting may occur in the organic electrolyte and the heat resistance thereof due to the swelling and melting may be observed.
Japanese Patent Application Nos. 2003-015766 and 2004-085059 disclose that water is used as a solvent at the time of an application process. However, in the case of using this water-soluble polymer, since the polymer itself has high affinity with water, the application layer may contain a large amount of moisture that may affect performances of the battery therein even after drying. This micro-porous composite film having a high moisture content may deteriorate the entire performance of the battery, such as cycle characteristics of the battery and long-term storage characteristics. Moreover, in the case of using the water-soluble polymer alone, the adhesive strength between the micro-porous polyolefin film and an inorganic material is not sufficient, which may cause problems during a battery assembling process and problems of stability inside the battery.
Japanese Patent Application No. 2004-227972 discloses that carboxyl methyl cellulose (CMC), which is a water-soluble polymer binder, is used to manufacture a separator for a non-aqueous electrolyte secondary battery. However, the long-term lifespan and cycle characteristics thereof may be deteriorated due to high affinity of the CMC with respect to moisture. Particularly, when the CMC is applied on the micro-porous polyolefin film, the surface tension value of an aqueous CMC solution is high, which may cause application thereof to be difficult. In order to solve this problem, a surfactant lowering surface tension necessarily be used. However, since the possibility of occurrence of an electrochemical sub-reaction due to the surfactant is high, the CMC requires special care. Moreover, since the CMC is easily broken when being mixed with the inorganic material due to deformation externally occurring, the application film thereof may be cracked or separated due to deformation resulting from the battery assembling process or the like, and this may affect performances and stability of the battery.